1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and in particular to a liquid crystal display apparatus preferably used for mobile information terminals (for example, PDAs), cellular phones, vehicle-mountable liquid crystal display apparatuses, digital cameras, personal computers, amusement devices, TVs and the like.
2. Description of the Related Art
Recently, liquid crystal display apparatuses are widely used, owing to their features of being thin and low in power consumption, for information devices such as notebook personal computers, cellular phones, electronic personal organizers and the like, and also for camera-integrated VTRs having a liquid crystal monitor.
As a display mode for realizing a high contrast and a wide viewing angle, a vertical orientation mode using a vertical orientation type liquid crystal layer has attracted attention. A vertical orientation type liquid crystal layer is generally formed using a vertical alignment layer and a liquid crystal material having a negative dielectric anisotropy.
For example, Japanese Laid-Open Patent Publication No. 6-301036 discloses a liquid crystal display apparatus having improved viewing angle characteristics. This is realized by generating an inclined electric field around openings provided in a counter electrode, which faces pixel electrodes with a liquid crystal layer interposed therebetween, and orienting liquid crystal molecules in an inclined manner around liquid crystal molecules which are in a vertically oriented state in the opening.
However, the structure disclosed in Japanese Laid-Open Patent Publication No. 6-301036 has the following problem. It is difficult to generate an inclined electric field in the entire area in the pixels. As a result, response of the liquid crystal molecules to a voltage application is delayed in some of the areas in the pixels, which causes an afterimage phenomenon.
In order to solve this problem, Japanese Laid-Open Patent Publication No. 2000-47217 discloses a liquid crystal display apparatus having a plurality of liquid crystal domains exhibiting a radially inclined orientation in pixels. This is realized by regularly arranging openings in pixel electrodes or a counter electrode.
Japanese Laid-Open Patent Publication No. 2003-167253 discloses a technology of regularly arranging a plurality of convexed portions in pixels, and thus stabilizing the orientation state of liquid crystal domains exhibiting a radially inclined orientation which appears around the convexed portions. This publication discloses that display characteristics can be improved by regulating the orientation of liquid crystal molecules using an inclined electric field caused by the openings provided in electrodes as well as by the regulating force provided by the convexed portions.
Recently, a liquid crystal display apparatus capable of high quality display both indoor and outdoor has been proposed (see, for example, Japanese Patent No. 2955277 and U.S. Pat. No. 6,195,140). Such a liquid crystal display apparatus, which is referred to as a transflective liquid crystal display apparatus, includes a reflective area and a transmissive area in a pixel. In the reflective area, images are displayed in a reflective mode, and in the transmissive area, images are displayed in a transmissive mode.
Currently commercially available transflective liquid crystal display apparatuses use an ECB mode or a TN mode, for example. Japanese Laid-Open Patent Publication No. 2003-167253 discloses a structure by which the vertical orientation mode is applied to a transflective liquid crystal display apparatus in addition to a structure by which the vertical orientation mode is applied to a transmissive liquid crystal display apparatus. Japanese Laid-Open Publication No. 2002-350853 discloses a technology for controlling the orientation of liquid crystal molecules (polyaxial orientation) in a transflective liquid crystal display apparatus including a vertical orientation type liquid crystal layer. The orientation is controlled by a convexed portion formed in an insulating layer, which is provided in order to make the thickness of the liquid crystal layer in the transmissive area twice as great as the thickness of the liquid crystal layer in the reflective area. The convex portion has the shape of, for example, a regular octagon. In the structure disclosed in this publication, a projection (concaved portion) or a slit (electrode opening) is formed at a position facing the convexed portion with the liquid crystal layer interposed therebetween. (See, for example, Japanese Laid-Open Publication No. 2002-350853, FIGS. 4 and 16).
Japanese Laid-Open Publications Nos. 2003-167253 and 2003-315083 describe a technology for realizing a radially inclined orientation of liquid crystal molecules in liquid crystal domains by forming convexed portions on a surface of a substrate closer to the side of the liquid crystal layer. It is also described that the radially inclined orientation is stabilized in the state where a voltage is applied, by the effect provided by the convexed portions formed on the substrate and by the orientation regulating structure provided in the other substrate.
According to the technology disclosed by Japanese Laid-Open Publications Nos. 2000-47217 and 2003-167253, convexed portions or openings are provided in pixels to form a plurality of liquid crystal domains (i.e., to divide each pixel into a plurality of areas), so that the orientation regulating force on the liquid crystal molecules is strengthened. A study performed by the present inventor has found that in order to obtain a sufficient orientation regulating force, it is necessary to form an orientation regulating structure such as convexed portions, openings or the like on both sides of the liquid crystal layer (i.e., regions which are on surfaces of a pair of substrates facing each other, the surfaces being closer to the liquid crystal layer). Such a structure complicates the production process. An orientation regulating structure provided in the pixels may reduce the effective numerical aperture of the pixels, or reduce the contrast ratio due to light leakage from an area around the convexed portions in the pixels. In the case where the orientation regulating structure is provided on both sides of the liquid crystal layer, the reduction in the effective numerical aperture and/or the reduction in the contrast ratio is more conspicuous because of the influence of the alignment margin of the substrates.
With the technology disclosed by Japanese Laid-Open Patent Publication No. 2002-350853, it is necessary to provide convexed portions or electrode openings on the opposite side to the concaved portions, which are provided for controlling the polyaxial orientation. This causes the same problems as described above.
With the technology disclosed by Japanese Laid-Open Patent Publication No. 2000-47217, 2003-167253, 2002-350853 or 2003-315803, there are the following problems even when the orientation regulating structure is provided in both of the substrates: response to gray scale display is slow; and/or it takes a long time until an afterimage, generated when a panel surface is pressed, disappears. It is difficult to use such technology for mobile-use liquid crystal display apparatuses.
The present invention, for solving the above-described problems, has an objective of providing a liquid crystal display apparatus having a plurality of radially inclined orientation domains in a pixel, which sufficiently stabilizes the orientation of liquid crystal molecules and thus realizes display quality equivalent to, or higher than, the display quality provided by the conventional apparatuses, with a relatively simple structure of having an orientation regulating structure for a radially inclined orientation in only one substrate.
Another objective of the present invention is to provide a liquid crystal display apparatus which further stabilizes the orientation of the liquid crystal molecules, and realizes a faster response in gray scale display or requires a shorter time until an afterimage, generated when the panel surface is pressed, disappears.